When a highly relativistic electron is injected off-axis into an ion channel, the restoring force of the radial field of the
ions will cause the electron to accelerate towards the axis, overshoot, and begin to undergo oscillations about the ioncolumn
axis at a characteristic frequency; the betatron frequency. This so-called betatron motion will cause the electron
to radiate hard x-rays in the forward direction. In two recent experiments at the Stanford Linear Accelerator Center
(SLAC), betatron x-rays in the 1-20kV range and in the 1-50MV range were produced with an electron beam with an
energy of 28.5 GeV for ion densities of about 1 x 1014 cm-3 and 1 x 1017cm-3, respectively. To make such an x-ray source
more compact, the 3km long SLAC linac would be replaced by a source of electrons from a Laser Wakefield accelerator
(LWFA). To increase the efficiency of converting laser into photons at high photon energies, we propose adding a
second stage where the LWFA electrons radiate via a second ion channel, independent of the accelerating process. This
two stage concept allows one to control the critical frequency of the emitted radiation as well as the efficiency of the
process.

Recent years have seen rapid improvement in the quality of electron beams produced by wakefields in plasmas.
The electron beams produced have inherently short durations and high peak current. To further shorten the pulse
duration of these beams for future applications, an experiment is proposed to produce a single sub-femtosecond
slice of electrons via an Inverse Free Electron Laser interaction (IFEL) with a few cycle laser pulse. The IFEL
is followed by a combined function, permanent magnet quadrupole triplet chicane that both disperses the beam
transversely while simultaneously focusing, allowing for efficient energy collimation to select the attosecond slice.
Simulations are presented showing the expected electron slice characteristics.

The relativistic Doppler effect offers a fundamental means of frequency upconverting electromagnetic radiation.
In 1993, Esarey et al.1 mentioned the possibility of scattering light at fast moving electrons to upconvert its
frequency. For the process to be efficient, one needs to have a highly compressed bunch of electrons, since only
then the scattering process can become coherent. The condition for coherency is, that the scale length of the
electron bunch or its density gradient needs to be on the order of the wavelength to be generated or smaller. This
is much tinier than what can be reached by commonly known techniques, including conventional accelerators as
well as laser-plasma accelerators.
Therefore, electrons are extracted from a small droplet or a thin foil by a highly relativistic driver laser
(a0 = eA0/mc2 &gap;&gap; 1). The electron bunch becomes accelerated and at the same time compressed by the forces of
the laser field. The acceleration can be achieved either by the relativistic ponderomotive force of a conventional
laser pulse, as suggested in,6 or by the longitudinal field on the optical axis of a radially polarized pulse, as
suggested in.8 In both cases, the bunch is compressed because of the fundamental snowplough effect of the
co-moving force, i.e. the laser pulse. Spacecharge forces are counteracting the compression, thus limiting the
amount of charge to be compressed. Nevertheless, in a wide range of parameters the edges of the electron bunches
density profile remain sharp, enabling coherent Thomson scattering.
We use analytic models and PIC simulations to describe and analyze thoroughly the effects occurring and
finally estimate the conversion efficiency that can be reached by this scheme. Techniques to increase the efficiency
and gain further control over the generated radiation are suggested and discussed. Reaching best possible control
over temporal envelope of the driver pulse appears to be the most important issue here.

The transverse emittance is an important parameter governing the brightness of an electron beam. Here we
present the first pepper-pot measurement of the transverse emittance for a mono-energetic electron beam from a
laser-plasma wakefield accelerator, carried out on the Advanced Laser-Plasma High Energy Accelerators towards
X-Rays (ALPHA-X) beam line. Mono-energetic electrons are passed through an array of 52 μm diameter holes in
a tungsten mask. The pepper-pot results set an upper limit for the normalised emittance at 5.5 ± 1 π mm mrad
for an 82 MeV beam.

Compact tuneable sources of ultrashort hard x-ray pulses can be realized by Thomson scattering, taking advantage
of the comparatively short wavelength of a scattered laser pulse with respect to the period length of
conventional undulators. Here, we present a detailed analysis and optimization of the efficiency of linear and
non-linear Thomson scattering when the process is driven with relativistic laser pulses and when the conventional
accelerator is replaced by a laser-plasma wakefield accelerator.

An ultra-bright high-power X-ray and γ-ray source is proposed. A relativistic flying mirror reflects a counterpropagating
electromagnetic radiation causing its frequency multiplication and intensification, while the role of
the mirror is played by a solid-density thin plasma slab accelerating in the radiation pressure dominant regime.
Frequencies of high harmonics generated at the flying mirror by a relativistically strong counter-propagating
radiation undergo multiplication with the same factor as the fundamental frequency of the reflected radiation,
approximately equal to the quadruple of the square of the mirror Lorentz factor. The theory of the reflectivity
of a moving thin plasma slab is presented.

Photon Landau damping of electron plasma waves with relativistic phase velocity has
been rst considered in 1997, in the frame of geometric optics [1]. Here we consider more
recent results based on a kinetic model where photon recoil is taken into account [2]. By
photon recoil we mean the change of a nite amount of momentum by photons, due to
the emission or the absorption of electron plasma waves. This gives a surprising quantum
avor to a purely classical description. Our approach is based on an exact form of the
wave kinetic equation. Kinetic and uid regimes of photon beam instabilities, and their
relevance to particle acceleration and new radiation sources are discussed. Quasi-lnear
results leading to a photon Boltzmann equation are also discussed. Diusion in the photon
momentum space are shown to be a consequence of photon-plasmon collisions, taken in the
geometric optics limit. A brief discussion of photon trapping by the plasma wave potential
is also included. Our theoretical discussion will be illustrated with the description of
recent experimental results using intense laser plasma interactions, as well as with a new
experimental proposal.

The temporal characteristics of the harmonic emission from solid targets irradiated with intense laser pulses is
examined in detail. In the case where the CoherentWake Emission mechanism is dominant it is found that indeed
the harmonics thus produced possess a frequency chirp resulting in non Fourier-Transform-Limited pulses. A
simple model explains the underlying physics while Particle-In-Cell simulations support the conclusions drawn.

When an intense ultrashort laser pulse impinges on an initially-solid target, it creates a dense plasma at the surface,
which reflects a large fraction of the incident light. At high enough intensities, high-order harmonics of the incident laser
frequency, associated in the time domain to trains of attosecond pulses, are generated in the light beam specularly
reflected by this "plasma mirror". The mechanisms leading to this generation are now relatively well-established, and the
first experimental evidence for attosecond pulses generated on plasma mirrors has recently been reported. An accurate
characterization of the temporal structure of the light reflected by plasma mirrors, down to the attosecond scale, however
remains an experimental challenge. In this paper, we describe three different methods that could be used for such
temporal measurements, from the femtosecond to the attosecond time scale. Two of them are interferometric techniques
which only require measurements of photons, while the third one is a new configuration of a now well-established
method, developed for attosecond pulses generated in gases, and based on photoelectron spectroscopy.

The interaction of relativistically intense (Iλ2>>1.3 1018Wcm-2μm2) laser pulses with a near step-like plasma density
profile results in relativistic oscillations of the reflection point. This process results in efficient conversion of the incident
laser to a phase-locked high harmonic spectrum, which allows the generation of attosecond pulses and pulse trains.
Recent experimental results on efficiency scaling, highest harmonic generated and beam quality suggest that very high
focused intensities can be achieved opening up the possibility of ultra-intense attosecond X-ray interactions for the first
time.

We study the possibility of producing short-wavelength magnetostatic structures in plasmas by exciting a plasma
magnetic mode in the collision of light pulses with relativistic ionization fronts. Results from PIC simulations
demonstrate the generation of these structures with existing state-of-the-art laser systems. We analyze the feasibility of
using the magnetic structure associated with the plasma magnetic mode as an undulator for compact synchrotron
radiation sources, illustrating the generation of ultrashort-wavelength radiation.

Electromagnetic wave generation in the extreme ultraviolet (XUV) and infrared (IR) wavelength range occurs
during the interaction of intense short laser pulses with underdense plasmas. XUV pulses are generated through
laser light reflection from relativistically moving electron dense shells (flying mirrors). A proof-of-principle and
an advanced experiment on flying mirrors are presented. Both of the experiments demonstrated light reflection
and frequency upshift to the XUV wavelength range (14-20 nm). The advanced experiment with a head-on
collision of two laser pulses exhibited the high reflected photon number. IR radiation, which is observed in the
forward direction, has the wavelength of 5 μm and dominantly the same polarization as the driving laser. The
source of the IR radiation is attributed to emission from relativistic solitons formed in the underdense plasma.

An analytical study of significant photon acceleration (frequency up-shift) in a plasma density wake produced by
two laser pulses in the mildly relativistic and linearized regime is presented. The wake amplitude is amplified and its
phase controlled using two coaxially, co-propagating laser pulses, which are considered to be identical but separated by a
fixed time. A third probe pulse, with a variable delay, is considered as "test particle" or quasi-photon propagating
through the amplified density wake, which experiences significant photon acceleration because of the local temporal and
spatial variation of the permittivity. The evolution of the "photon" is studied using Hamiltonian theory. The significant
frequency up-shift is much larger than that produced by the wake of a single relativistic laser pulse in the highly
relativistic nonlinear wake regime. Our study demonstrates that the inter-pulse separation between the "controlling"
pulse and the "driver" pulse, producing the amplified density wake, can provide an additional degree of freedom for
tuning the maximum up-shift of the probe photon frequency.

The role of thermal effects on Raman amplification are investigated. The direct effects of damping on the
process are found to be limited, leading only to a decrease from the peak output intensity predicted by cold
plasma models. However, the shift in plasma resonance due to the Bohm-Gross shift can have a much larger
influence, changing the required detuning between pump and probe and introducing an effective chirp through
heating of the plasma by the pump pulse. This "thermal chirp" can both reduce the efficiency of the interaction
and alter the evolution of the amplified probe, avoiding the increase in length observed in the linear regime
without significant pump depletion.
The influence of this chirp can be reduced by using a smaller ratio of laser frequency to plasma frequency,
which simultaneously increases the growth rate of the probe and decreases the shift in plasma resonance. As
such, thermal effects only serve to suppress the amplification of noise at low growth rates. The use of a chirped
pump pulse can be used to suppress noise for higher growth rates, and has a smaller impact on the peak output
intensity for seeded amplification.
For the parameter ranges considered, Landau damping was found to be negligible, as Landau damping rates
are typically small, and the low collisionality of the plasma causes the process to saturate quickly.

Energy Energy transfer between a long (3-10 ps) "pump" pulse and a short (400 fs) "seed" one, both at a wavelength of 1.057μm
quasi counterpropagating in an underdense preformed plasma and produced from the ionization of a gas jet, was
observed. Numerical simulations reveal that the energy transfer is due to the coupling involving ion acoustic waves
excited in the Stimulated Brillouin Backscattering in the strong coupling regime. The plasma characteristics were
tailored using a high-energy ionization laser beam and the plasma density was controlled using a Thomson scattering
diagnostic. The energy exchange was observed for different gas (ion) types, pressures (plasma densities), polarization
and intensities of the interacting beams.

The nonlinear regime of Raman amplification has been studied including the combined effects of relativistic and
ponderomotive nonlinearities. The study is important for interaction of mildly relativistic pump and probe laser pulses.
Nonlinear coupled temporal evolution of fields and density in Raman amplification is analyzed. It is shown that the
saturation amplitude and time of the probe pulse in nonlinear regime depends upon the intensity of the electromagnetic
waves and the density of the medium. Further in the nonlinear regime the probe laser pulse gain is severely affected by
changes in both the electromagnetic wave amplitude and the plasma density.

Stimulated Raman backscattering in plasma has been suggested as a way to amplify short laser pulses to intensities
not limited by damage thresholds as in chirped pulse amplification using conventional media. Energy
is transferred between two transverse electromagnetic waves, pump and probe, through the parametric interaction
with a longitudinal Langmuir wave that is ponderomotively excited by their beat wave. The increase of the
plasma temperature due to collisional absorption of the pump wave modifies the dispersion of the Langmuir wave:
firstly, its resonance frequency rises (Bohm-Gross shift), and secondly, Landau damping sets in. The frequency
shift acts in a similar way to a chirp of the pump frequency, or a density ramp: different spectral components of
the probe satisfy the resonance condition at different times. This limits their growth, while increasing the bandwidth
of the amplifier, thus leading to superradiant amplification. Landau damping may shorten the probe pulse,
but reduces the amplification efficiency. We investigate these effects analytically and using numerical simulations
in order to assess their importance in experimental demonstrations, and the possibility of applications.

Raman backscattering (RBS) in plasma is an attractive source of intense, ultrashort laser pulses, which has the
potential asa basic for a new generation of laser amplifiers.1 Taking advantage of plasma, which can withstand
extremely high power densities and can offer high efficiencies over short distances, Raman amplification in
plasma could lead to significant reductions in both size and cost of high power laser systems. Chirped laser pulse
amplification through RBS could be an effective way to transfer energy from a long pump pulse to a resonant
counter propagating short probe pulse. The probe pulse is spectrally broadened in a controlled manner through
self-phase modulation. Mechanism of chirped pulse Raman amplification has been studied, and features of
supperradiant growth associated with the nonlinear stage are observed in the linear regime. Gain measurements
are briefly summarized. The experimental measurements are in qualitative agreement with simulations and
theoretical predictions.

High power short pulse lasers are usually based on chirped pulse amplification (CPA), where a frequency chirped
and temporarily stretched "seed" pulse is amplified by a broad-bandwidth solid state medium, which is usually
pumped by a monochromatic "pump" laser. Here, we demonstrate the feasibility of using chirped pulse Raman
amplification (CPRA) as a means of amplifying short pulses in plasma. In this scheme, a short seed pulse is
amplified by a stretched and chirped pump pulse through Raman backscattering in a plasma channel. Unlike
conventional CPA, each spectral component of the seed is amplified at different longitudinal positions determined
by the resonance of the seed, pump and plasma wave, which excites a density echelon that acts as a "chirped"
mirror and simultaneously backscatters and compresses the pump. Experimental evidence shows that it has
potential as an ultra-broad bandwidth linear amplifier which dispenses with the need for large compressor
gratings.

The dynamics of relativistic electrons in a laser driven plasma cavity are studied via measurements of their
radiation. For ultrashort laser pulses at comparatively low focused laser intensities (3 < a0 < 10), low density
and long f-number of 10, electrons are predominantly accelerated in the wakefield leading to quasi-monoenergetic
collimated electron beams and well collimated (< 12 mrad) beams of comparatively soft x-rays (1-10 keV) with
unprecedented small source size (2-3 μm). For laser pulses with increasing laser intensity (10 < a0 < 30),
density and short f-number (< 5), electrons are accelerated directly by the laser, leading to divergent quasimaxwellian
electron beams and divergent (50-95°) beams of hard x-rays (20-50 keV) with relatively large source
size (> 100 μm). In both cases, the measured x-rays are well described in the synchrotron asymptotic limit of
electrons oscillating in a plasma channel. At low laser intensity transverse oscillations are small as the electrons
are predominantly accelerated axially by the laser generated wakefield. At high laser intensity, electrons are
directly accelerated by the laser. A betatron resonance leads to a tenfold increase in transverse oscillation
amplitude and electrons enter a highly radiative regime with up to 5% of their energy converted into x-rays.

In 3D simulations, PIC codes cannot resolve the radiation of short wavelength compared to the grid spacing,
which raises challenges in multi-dimensional simulations because of memory constraints. However, in many
plasma physics scenarios (e.g. laser wakefield acceleration) the radiation mechanisms can cover several orders of
magnitude in energy/frequency (from the THz range, associated with transition radiation of relativistic electron
beams, to gamma-rays, associated with the betatron radiation of self-injected electrons in the bubble or blow-out
regime). We describe a massivelly parallel post-processing radiation diagnostic that takes the track information
from 3D/2D particle-in-cell simulations and determines the full radiation spectrum of the corresponding particle(
s). Benchmark examples with cyclotron/synchrotron radiation as well as betatron radiation are presented
and compared with the analytical predictions. Special emphasis is given to the numerical properties of the
diagnostic, in particular the resolution of the particle tracks, the diagnostic spectral and spatial resolutions, as
well as the different aproximations on the numerical calculation of the radiation integral over the trajectory of
the particles. We then use this diagnostic to probe different scenarios, taking advantage of the spatial, temporal
and frequency resolved characteristics of the diagnostic.

In this paper, we present the first temporal characterization of betatron X-ray radiation. Results obtained
from time resolved x-ray diffraction experiments, for which the ultra fast phase transition of non thermal melting
of InSb was used, indicates that the x-ray pulse duration is less than 1 ps. We then propose a novel technique
to improve the spectral and flux properties of the x-ray source. The energy and the flux can be enhanced when
the electron beam propagates and oscillates in a tailored plasma density profile.

The interaction of ultrashort intense laser pulses with plasma can produce electromagnetic radiation of ultra-broad
spectra ranging from terahertz (THz) radiation to keV x-rays and beyond. Here we present a review of our recent
theoretical and numerical investigation on high power THz generation from tenuous plasma or gas targets irradiated by
ultrashort intense laser pulses. Three mechanisms of THz emission are addressed, which include the linear mode
conversion from laser wakefields in inhomogeneous plasma, transient current emission at the plasma-vacuum
boundaries, and the emission from residual transverse currents produced by temporally-asymmetric laser pulses passing
through gas or plasma targets. Since there is no breakdown limit for plasma under the irradiation of high power lasers, in
principle, all these mechanisms can lead to terahertz pulse emission at the power of beyond megawatt with the field
strength of MV/cm, suitable for the study of high THz field physics and other applications.

Electromagnetically induced transparency (EIT) is a well-known quantum phenomena that electromagnetic wave
controls the refractive index of medium. It enables us to create a passband for low frequency electromagnetic wave
in a dense plasma even if the plasma is opaque for the electromagnetic wave. This technique can be used to prove the
ion acoustic wave because the ion acoustic frequency is lower than the plasma frequency. We have investigated a
feasibility of electromagnetic radiation at THz region corresponding to the ion acoustic frequency from a dense
plasma. We confirmed that the passband is created at about 7.5 THz corresponding to the ion acoustic frequency in
the plasma (1021 cm-3) with a Ti:Sapphire laser (800 nm, 1017 W/cm2). The estimated radiation power is around 1
MW, which is expected to be useful for nonlinear THz science and applications.

Terahertz (THz) radiation from the interaction of ultrashort laser pulses with gases is studied both theoretically and experimentally.
We theoretically study the THz generation based on transient ionization current model and give the relation
between the final THz field and the initial transient ionization current. Recent experimental results on optimization of THz
radiation in laser air interaction are also shown. We find by use of a simple aperture to change the laser field distribution,
the terahertz wave amplitudes can be enhanced by more than eight times than those of aperture-free cases. We use two
dimensional particle-in-cell codes to simulate the experiments and give possible explanations.

The Advanced Laser-Plasma High-Energy Accelerators towards X-rays (ALPHA-X) programme is developing laserplasma
accelerators for the production of ultra-short electron bunches with subsequent generation of incoherent radiation
pulses from plasma and coherent short-wavelength radiation pulses from a free-electron laser (FEL). The first
quantitative measurements of the electron energy spectra have been made on the University of Strathclyde ALPHA-X
wakefield acceleration beam line. A high peak power laser pulse (energy 900 mJ, duration 35 fs) is focused into a gas jet
(nozzle length 2 mm) using an F/16 spherical mirror. Electrons from the laser-induced plasma are self-injected into the
accelerating potential of the plasma density wake behind the laser pulse. Electron beams emitted from the plasma have
been imaged downstream using a series of Lanex screens positioned along the beam line axis and the divergence of the
electron beam has been measured to be typically in the range 1-3 mrad. Measurements of the electron energy spectrum,
obtained using the ALPHA-X high resolution magnetic dipole spectrometer, are presented. The maximum central energy
of the monoenergetic beam is 90 MeV and r.m.s. relative energy spreads as low as 0.8% are measured. The mean central
energy is 82 MeV and mean relative energy spread is 1.1%. A theoretical analysis of this unexpectedly high electron
beam quality is presented and the potential impact on the viability of FELs driven by electron beams from laser
wakefield accelerators is examined.

Electron acceleration using plasma waves driven by ultra-short relativistic intensity laser pulses has
undoubtedly excellent potential for driving a compact light source. However, for a wakefield accelerator to
become a useful and reliable compact accelerator the beam properties need to meet a minimum standard. To
demonstrate the feasibility of a wakefield based radiation source we have reliably produced electron beams
with energies of 82±5 MeV, with 1±0.2% energy spread and 3 mrad r.m.s. divergence using a 0.9 J, 35 fs 800
nm laser. Reproducible beam pointing is essential for transporting the beam along the electron beam line. We
find experimentally that electrons are accelerated close to the laser axis at low plasma densities. However, at
plasma densities in excess of 1019 cm-3, electron beams have an elliptical beam profile with the major axis of
the ellipse rotated with respect to the direction of polarization of the laser.

Focussing ultra-short electron bunches from a laser-plasma wakefield accelerator into an undulator requires
particular attention to be paid to the emittance, electron bunch duration and energy spread. Here we present
the design and implementation of a focussing system for the ALPHA-X beam transport line, which consists of
a triplet of permanent magnet quadrupoles and a triplet of electromagnetic quadrupoles.

We reported the production of a plasma channel in a capillary discharge-produced plasma. Plasma parameters of its
channel were observed by use of both a laser interferometer and a hydrogen plasma spectrum. A time-resolved
electron temperature was measured, and its maximum temperature of 3 eV with electron densities of the order of
1017 cm-3 was observed at a discharge time of 150 ns and a maximum discharge current of 400 A. Intense laser pulse
was guided over many vacuum Rayleigh lengths using its channel.

Pulse compression through filamentation in a free-space argon gas-filled cell has been demonstrated by use of the
high energy laser pulse. Compression and splitting of the optical laser pulse due to multiple filamentation in an argon
gas-filled cell were observed. A 130-fs pulse was compressed to less than 60 fs (full width at half-maximum) with
the output energy of 16 mJ at the argon gas pressure of 25 kPa.

A wide-band spectral diagnostic system based on dispersion property of the Zinc Selenide prism, a crystalline
material highly dispersive in the near-to-far infrared spectral range, has been studied and developed for the laser
wakefield acceleration experiment at LOA for the measurement of few femto-seconds long electron beam. The
extensive PIC simulation studies of the colliding-beam LWFA have shown very short electron beam duration of
less than 10 femtoseconds. The prism spectrometer diagnostic with highly sensitive Mercury Cadmium Telluride
infrared detector and the diffraction-grating spectrometer with a high-resolution imaging visible camera together
have the spectral range coverage and resolution capable of detecting ultra-short Coherent Transition Radiation
(CTR) generated by interaction of bunch charges with a 100 microns thickness aluminum foil. The beam profile
of asymmetric shape then could be extracted from the CTR spectrum by inverse Fourier transformation with
Kramers-Kronig relation. The diagnostic system has been tested and calibrated for characterization of blackbody
source spectrum and spectral responsivity. The measurement of electron beam duration of few femtoseconds has
yet been convincingly shown with high resolution, and the measurements of this kind have important implication
in understanding and subsequent successful operation of the future FEL light source with a highly mono-energetic
LWFA beam source.